1. Field of the Invention
One embodiment of the present invention relates to a power storage device. Note that the power storage device refers to all elements and devices that have a function of storing power. Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, an imaging device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
In recent years, a variety of power storage devices, for example, nonaqueous secondary batteries such as lithium-ion batteries (LIBs), lithium-ion capacitors (LICs), and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for the uses of electric appliances, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; and next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary batteries are essential for today's information society as rechargeable energy supply sources.
Widely-used lithium-ion secondary batteries generally use a nonaqueous electrolyte (also referred to as a nonaqueous electrolyte solution or simply an electrolyte solution); the nonaqueous electrolyte contains an organic solvent such as ethylene carbonate, propylene carbonate, fluorinated cyclic ester, fluorinated acyclic ester, fluorinated cyclic ether, or fluorinated acyclic ether, and a lithium salt containing lithium ions. Note that the fluorinated cyclic ester in this specification refers to a cyclic ester in which fluorine is substituted for hydrogen as in a cyclic ester having alkyl fluoride. Similarly, in the fluorinated acyclic ester, the fluorinated cyclic ether, or the fluorinated acyclic ether, fluorine is substituted for hydrogen.
However, the organic solvent has volatility and a low flash point; thus, when the organic solvent is used in a lithium-ion secondary battery, the internal temperature of the lithium-ion secondary battery might increase owing to internal short-circuit, overcharging, or the like, and the lithium-ion secondary battery would explode or catch fire. In addition, some kinds of organic solvent produce a hydrofluoric acid by a hydrolysis reaction. This hydrofluoric acid corrodes metal, which might decrease the reliability of batteries.
In view of the above problems, an ionic liquid which has non-volatility and non-flammability has been used as a nonaqueous solvent for a nonaqueous electrolyte of a lithium-ion secondary battery. Examples of such an ionic liquid are an ionic liquid containing an ethylmethylimidazolium (EMI) cation and an ionic liquid containing an N-methyl-N-propylpiperidinium (PP13) cation (see Patent Document 1).